In recent years, with realization of image in digital form and progress of picture compression technology, various picture compression encoding processing have been carried out. However, in the prior art, code quantity allocated to picture data of a predetermined unit is fixed to a certain fixed quantity (in the case of the fixed rate), or even in the case of a variable rate to vary code quantity allocated to picture data of a predetermined unit in dependency upon fineness of picture, code quantity allocated at the maximum has been already determined. Therefore, the maximum value of code quantity within a predetermined time is determined. As a result, when continuous complicated pictures are successive, there was limitation in the code quantity. As a matter of course, increase or decrease of code quantity allocated to the picture leads to degree of the picture quality. In view of the above, in the prior art, it has been carried out to control code quantity so that the picture quality deterioration at the time of decoding becomes minimum while taking the same code quantity as the entirety of a single picture by a method of allocating a larger code quantity to complicated portions on the picture, etc.
However, way of sensing of the picture quality deterioration of viewer (viewer/listener) is changed to what portion attention is drawn within one picture. On the other hand, the portion that the viewer carefully observes is the portion important within that picture, and that portion is not limited to the complicated portion within that picture. For this reason, with the method of allocating a larger code quantity to the complicated portion on picture, inconvenience as described below takes place. Namely, in the case where, e.g., the main person exists within the complicated background, etc., larger codes are allocated to the complicated background. As a result, the code quantity allocated to the main person that the viewer carefully observes is less. Thus, the viewer feels picture quality deterioration at the time of decoding.
To cope with this, it is conceivable that the operator is permitted to arbitrarily change, every picture, allocation of code quantity within the picture. However, it is necessary to take much labor in order to allow the operator to set area different in allocation of code quantity, and in situations where real time characteristic such as live broadcasting, etc. is required, such an operation is impossible.
Meanwhile, in the Japanese Patent Application Laid Open No. 44110/1995 publication, there is disclosed the technology in which position of visual point of user is detected to display a predetermined area in the vicinity of the visual point position within one picture by high resolution, and to display other area by low resolution. In other words, it can be said that this technology is the technology in which code quantity allocated to the area in the vicinity of the visual point position within one picture is increased and code quantity allocated to other area is decreased. In accordance with this technology, picture quality deterioration that the user feels at the time of decoding can be suppressed while encoding respective pictures on the real time basis within a limited code quantity.
However, in the case where such an approach is always employed to increase code quantity allocated to the area in the vicinity of the visual point and to decrease code quantity allocated to other area, even if other area is not so complicated and code quantity to be allocated is not decreased, even in the case where quantity of codes generated is not so many, i.e., there is margin in the code quantity, there is the problem that quantity of codes allocated to other area is necessarily reduced and the picture quality of other area is unnecessarily deteriorated.
Moreover, the size of the area that the viewer carefully observes within picture is not limited to fixed size. Accordingly, in the case where the size of the area where quantity of codes allocated is increased is caused to be fixed (constant), problems as described below takes place. Namely, in the case where the area that the viewer carefully observes is larger than the area where code quantity is increased, larger code quantity is allocated in the area of the center side of the area that the viewer carefully observes so that there results small deterioration of the picture quality. In the area of the peripheral side of the area where the viewer carefully observes, code quantity to be allocated becomes small. As a result, deterioration of the picture quality becomes large. Thus, there results picture difficult to see for the viewer, i.e., picture of which picture quality has been deteriorated. To the contrary, in the case where the area where the viewer carefully observes is smaller than the area where the code quantity is increased, a large code quantity would be allocated also to the area except for the area where the viewer carefully observes. As a result, there may take place circumstances where code quantity allocated to the area where the viewer carefully observes becomes small.
This invention has been made in view of the problems as described above, and its object is to provide a picture compression encoding apparatus and a picture compression encoding method which are capable of compression-encoding picture data on the real time basis within a limited code quantity, and is permitted to obtain picture (image) having high picture quality from a viewpoint of visual sense for user as far as possible at the time of decoding.